Furnaces such as coal burning stoves remain a widely used method of space heating in both urban and rural areas. In order for a coal fire to burn, the fire must be provided with an adequate supply of oxygen (oxygenation). Natural draft is too unpredictable to serve as a reliable method of oxygenating a coal fire. A fan promotes oxygenation of the coal fire by one of two methods. In one method, the fan blows fresh air into the coal fire (forced draft). Forced draft fans typically blow air upward from beneath the coal fire to create a higher rate of burning and thus allow for the use of a cheaper, lower grade of coal. In a second method, the fan exhausts oxygen-poor air from the coal fire, consequently drawing in fresh air (induced draft). Induced draft fans typically are installed in a flue outlet and must be capable of withstanding the high temperatures of coal exhaust. In either application, the fans allow the operator to control the rate at which coal burns within the stove by controlling the amount of fresh air available to the coal fire.
A drive shaft of the fan is typically turned by a 120 volt alternating current (AC) motor. The AC motor is usually connected to an external primary power supply such as commercial power. It is widely known that commercial power supplies and distribution systems are prone to occasional malfunctions, especially in inclement weather. During the winter, when the need for heating is greatest, power outages prevent the fan from oxygenating the coal fire, causing the fire to bum itself out. This necessitates restarting the fire once power is restored, which is a cumbersome and time-consuming process. Depending on the duration of the power outage, occupants of the heated space are seriously inconvenienced, and in the extreme, the health of the occupants is adversely affected and damage occurs to water pipes and other temperature-sensitive areas of the space.
An early attempt to provide a backup motor system is disclosed in Welch, U.S. Pat. No. 1,601,934. A loss of power triggers brushes to contact a commutator in a DC powered backup motor. An unspecified electroresponsive means for triggering the backup power system is disclosed. In addition, the invention disclosed relies upon the mechanical connection of the primary motor and the backup motor. This is also an undesirable arrangement because the backup assembly impedes the efficient working of the primary motor assembly under normal conditions. Also, this method requires a special brush arrangement, thus precluding the use of an off-the-shelf motor.